The invention relates to a method for conveying sheets in a printing machine and to a device for carrying out the method.
In sheet-printing machines, which in the following text is understood in a very general sense to be devices for printing material in sheet form. The sheets which are to be printed are removed from a stack of paper and are then fed, via conveying devices, such as conveying suckers, sucker rollers, suction belts or grippers, to one or more successive printing stations, in which monochrome or multi-colored printed images are transferred to the sheets. The finished, printed sheets are then placed on a stack.
In rotary offset sheet-printing machines, the sheets are guided, held by grippers, from printing unit to printing unit alternately via impression cylinders and guide drums downstream of the cylinders, the impression cylinders, together with the associated blanket cylinders of a printing unit, forming the respective printing stations. Due to the high levels of precision required for the gripper devices and the remaining moving parts, the outlay on the device which is needed to produce high-quality multicolored printing in the rotary offset sheet-printing machines described is comparatively high.
Published, Non-Prosecuted German Patent Application DE-A 196 431 06 A1 discloses a printing machine for printing sheets, in which the sheets are conveyed to the individual printing stations on a revolving, electrostatically charged belt. A drawback of conveying sheets with the aid of electrostatic charging is that the electrostatic charges may result in voltage sparkovers, and that the electrostatic holding forces which are exerted on the sheets are comparatively low, so that the charge has to be applied over a large area in order to ensure that the sheets are conveyed in register. A further problem is that the finished, printed sheets stick together after they have been deposited on the stack of sheets, owing to residual electrostatic charges, and have to be separated again in order for them to be processed further.
Furthermore, U.S. Pat. No. 5,019,871 discloses a rotary printing machine in the form of an electro-photographic color copier, in which the toner of the colors black, yellow, cyan and magenta is transferred via a light-sensitive drum onto a sheet-guiding cylinder in the form of a transfer drum, on the circumferential surface of which the sheets which are to be printed are held by electrostatic charging.
It is accordingly an object of the invention to provide a method for conveying sheets in a printing machine and a device for carrying out the method which overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, in which the sheets are conveyed in register to the individual printing stations without it being necessary to use complex mechanical moving parts, such as gripper devices, etc.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for guiding sheets in a sheet-printing machine, which includes conveying sheets to printing stations in register with sheet-conveying devices, the sheets being joined to the sheet-conveying devices in a secure position by way of an adhesive.
A further object of the present invention is to provide a printing machine for carrying out the above-mentioned method, in which machine the sheets which are to be printed are guided in register with a high level of precision and at least partly without using mechanical holding devices or with assistance provided to the mechanical holding devices.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for conveying sheets in a printing machine and a device for carrying out the method, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.